64bit on PA-RISC is not recommended because 64bit binaries (using 64bit instructions) are generally larger than 32bit. The PA-RISC can execute 32bit instructions faster anyway.

The only real reason to go 64bit is if you have more than 4G of RAM - because with 32 bits, you can only address 4294967296 (2^32) bytes of memory. 64bits can address up to 16 exabytes, and I don't think there is a single computer with that much RAM.

basically,
32bit: faster, smaller, limited to 4GB ram
64bit: slower, larger, can use more than 4GB

This is PA-RISC only though. Architectures like amd64 execute 64bit code at about the same speed as 32bit code, and 64bit wouldn't be that much larger since it's a CISC arch.

HP for example, recommends to use 64bit HP-Ux with these systems. They recommend 256M and more for GUI mode.

gtbX, you have mentioned AMD64, it is CISC, but exectution core is RISC - every CISC instruction is divided into set of small RISC instructions. Theoretically, that means, after instructions are extracted, you will get the same size of code like on other RISC machines. I beleave that CISC instructions are actually macros of CPU's micro-code.
That means, 64bit software of RISC machine should use more disk space and RAM, but execution process should be faster because instructions are not extracted. Additionaly, more space should be required only for data, that means 64bit soft is larger than 32bit soft about twice.
For example on Gentoo installation CD there is vmlinux that takes 5.3M and vmlinux64 that takes 7.1M.
In Gentoo requirements, they write that 256M is ideal, so for me 512M should be good?
I have 3.5G RAM doesn't that mean that most of the time, I'll have a lot of free memory on 64bit Kernel?

AMD64, it is CISC, but exectution core is RISC - every CISC instruction is divided into set of small RISC instructions.

Yes, AMD64 is CISC externally, with a RISC core. I only brought up CISC (specifically variable length instructions) to argue that 64bit software isn't necessarily much larger than the size of 32bit software.

skywalker wrote:

That means, 64bit software of RISC machine should use more disk space and RAM, but execution process should be faster because instructions are not extracted.

Not necessarily. There are many things that can affect the execution speed of a CPU. RISC machines do not need to expand their instructions, but this is independent of the word size, whether 32 or 64. RISC machines can be microcoded as well, but it's generally a 1:1 mapping instead of 1:many. 64bit mode may be slower than 32bit simply because there are more bits to shuffle around. The instruction decoder may have more work to do in 64bit mode, or the ALU's don't work as efficiently or any number of other reasons. SPARC machines are also slower in 64bit mode - generally they use a 64bit kernel and 32bit user space (even Solaris does this). I don't know if it's possible to mix word sizes on HPPA though.

Can you use the PA-RISC 64bit kernel with less than 4GB of ram? I don't see why not. But unless you need to use 64bit instructions, need the extra addressing space (for physical or virtual memory) or are willing to take the performance hit, 32bit may be the better choice.